Insectosaurus was a one-inch (25.4 mm) grub transformed by radiation into a 350-foot (106.68 meters) tall monster that attacked Tokyo. She was captured in 1969 and formed a close bond with the Missing Link, who is the only person that can understand her.

Insectosaurus originally was nothing more than a simple grub with no unique characteristics, but after her mutation she donned an almost mammalian-like appearance similar to that of an orange hamster with a massive overbite, four pincer-like arms, antennas, two stubby feet, and a short reptilian-like tail. Insectosaurus is a massive creature in size even dwarfing the giant Ginormica. She is seen in the movie using her powerful pincers to pull the bridge back in place in the Golden Gate fight seen. Not only can she do that, she can also shoot sticky web substances from her nostrils that were used only on the Alien Robots eye to stop it from reacting for a short time so the other Monsters could get her and Ginormica (Susan Murphy) to the Golden Gate Bridge. Although of her big physique, she is actually a baby. When she walks, she does that in a dinosaur-like fashion and is very slow. Sometimes she doesn't know what to do so she is mesmerized and navigated by a helicopter carrying a light in front of Insectosaurus causing her to stare at it and follow where it goes. When that happens, the pupils of her eyes grow almost as big as the size of her eye making her look much scarier than she actually is.

After her metamorphosis near the end of the film, Insectosaurus' appearance is changed to that of a more butterfly-like creature (similar to Mothra of Godzilla fame) while still possessing her hamster-like face, furry body, massive overbite and reptilian-like tail. She's been renamed to Butterflyosaurus.